Add cargo keywords and categories.

Resolves #306. Suggestions welcome for better keywords/categories, though there is a limit of 5 of each.

Cargo.toml

@@ -3,7 +3,8 @@ authors = ["The Rust Project Developers"]
 description = "A collection of numeric types and traits for Rust, including bigint,\ncomplex, rational, range iterators, generic integers, and more!\n"
 documentation = "http://rust-num.github.io/num"
 homepage = "https://github.com/rust-num/num"
-keywords = ["mathematics", "numerics"]
+keywords = ["mathematics", "numerics", "bignum"]
+categories = [ "algorithms", "data-structures", "science", "cryptography" ]
 license = "MIT/Apache-2.0"
 repository = "https://github.com/rust-num/num"
 name = "num"